Pleasant weather belies urgent issue

The appearance of blooming daffodils and cherry trees in New Haven on New Year’s Day has led many to speculate about the possible link between the mild winter and global warming. But understanding the complex phenomenon of climate change and its implications requires a closer look at the various atmospheric processes that are warming the planet.

Now, geology and geophysics professor Steven Sherwood and his colleagues are making headway toward understanding those processes with cutting-edge geological research on atmospheric forces such as water vapor and cloud systems.

“Everyone acknowledges that warming is happening and that we’re causing it, but the debate is over how much of a problem it will be,” Sherwood said. “Clouds and water vapor are two particular atmospheric constituents that are … very important, owing to their ability to affect the amount of energy our planet absorbs from the sun and radiates back to space. Climate change might be highly sensitive to clouds and humidity, but we don’t know how sensitive exactly.”

The water vapor system is an important but little-understood player in climate change. When scientists understand the system better — in particular the typical heights of tropical storms and the amount of water they retain in the atmosphere — they will be able to make more accurate predictions about the impact of climate change, Sherwood said. Humidity in the atmosphere might be exacerbating the warming effects of the excess CO2, he said, because water itself is a greenhouse gas and acts in a positive feedback mechanism: When the climate becomes warmer, the amount of atmospheric water vapor increases, which in turn increases the amount of heat the atmosphere traps.

Sherwood’s lab also tackles questions about cloud behavior and physics, which, according to Sherwood, is the greatest source of uncertainty in our ability to predict climate change. His projects in this area include researching the heights clouds attain as well as their water content, solar reflectance, particle sizes and shapes, using models of individual clouds and the statistical properties of cloud populations.

Clouds may exert negative feedback since warming increases cloud cover, and thick clouds can reduce incoming solar infrared radiation. However, it is equally likely that clouds generate a positive feedback effect since they trap terrestrial infrared radiation. The net effect observed, Sherwood said, depends on the specific properties of a given cloud population.

Though the research is far from yielding concrete conclusions, one result is clear: The effects of human activity on climate change have yet to be fully seen.

“We’re only seeing the tip of the iceberg at this point,” Sherwood said. “The effect is certainly delayed … we’ve only experienced about half of the climate change we’re due for.”

The delay is caused by the thermal inertia of oceans, meaning that bodies of water of that size react slowly to changes in temperature.

The uncertainty of the extent of climate change makes it difficult for scientists to predict the phenomenon’s effects on components of the Earth’s biosphere such as the soil, plants and oceans. It is even conceivable that these individual components might introduce positive feedback loops of their own into the climate system, making the phenomenon even more complex, Sherwood said.

Sherwood also said the impact of human activity on the climate is close to irreversible.

“The options for undoing what we’ve done are probably unworkable,” Sherwood said. “We must be prepared to live with a different climate.”

At this late date, he said, the best option for reducing the future impact of carbon dioxide emission is carbon sequestration, a relatively new technology that has yet to catch on because of the high cost of implementation. Carbon sequestration, as its name implies, is the process of pumping and permanently trapping carbon dioxide exhaust from power plants and other industrial refineries into geological reservoirs and deep-sea sediments.

Although the process would add about 20 percent to the cost of generating power, Sherwood said, he thinks its positive effects would be worth the financial investment. One advantage to this approach is that it does not call for a radical change in our current behavior.

“Carbon sequestering allows us to go on doing what we’re doing – only be cleaner about it,” he said.

In a world powered almost exclusively by fossil fuels, Sherwood said he thinks the option is more practical than a complete shift to alternative fuel sources such as wind, nuclear or biogas. These technologies can only be administered on relatively small scales, he continued, and would not be able to accommodate fully our current energy needs.

Sherwood also cited geo-engineering as a feasible “last-resort” scenario. Among the geo-engineering proposals circulating among the scientific community is to fly a fleet of jets that would emit air pollution in the form of sulfur dioxide into the stratosphere. The pollution would condense into droplets and thus reduce the amount of sunlight reaching the Earth’s surface, Sherwood said.